The invention relates generally to polyester containers. In particular, the polyester container is formed of polyethelene terephthalate (PET) and provided with a base configuration which enables the container to resist deformation while being filled with a hot liquid. Furthermore, the base configuration also enables the container to resist deformation during internal pressure changes.
Typically, PET containers are formed by a process in which an injection molded tubular preform is heated and expanded to produce a semi-rigid, thin-walled container. During production and use, the container will be exposed to various pressures and forces. With this in mind, the container must be designed to withstand these physical influences while maintaining an aesthetically and commercially acceptable product.
During the process of molding a PET container, the preform is stretched and inflated (blown) so as to impart both axial and radial elongation into the material. In the art, such formation is known as biaxial elongation. However, biaxial elongation can also impart retractive stresses within the container. If not relaxed or physically restrained, the stresses may subsequently cause the container to shrink and deform into an aesthetically and commercially unacceptable product.
The influence of unrelaxed retractive stresses is of particular significance during various production phases of the container. Immediately after being demolded, the elevated temperature of the PET results in an article that is considerably less rigid than the final product. Predictably, the retractive stresses have a greater influence during this phase of production and the "memory" of the PET causes the container to partially return to its preform shape.
Another phase of production where retractive stresses are of significance occurs during "hot-fill" of the container. When a beverage or product at an elevated temperature is dispensed into the container, the beverage temperature imposes additional mechanical stresses on the container's structure. Immediately upon being dispensed into the container, the "hot" temperature of the beverage decreases the rigidity of the container and the container again becomes susceptible to the effects of the unrelaxed retractive stresses.
Beyond the production process the container must continue to maintain its ability to withstand deformation. For example, as a hot-filled liquid cools, its volume reduces and a resultant negative pressure is produced. In contrast, sudden increases in internal pressure can occur when the container is handled or dropped. In both instances the container must be capable of resisting deformation.
It is known that within a cross-section of the base of a PET container, the molecular orientation of the PET will not be uniform. Rather, the bottom wall will consist of several regions of varying molecular orientation. One region, an amorphous region, is located at the center of the base. Here, the PET remains significantly thick and is not stretched or oriented by the blowing process. The bottom wall of the container will also consist of a uniformly oriented region located adjacent to the peripheral edge of the base. In this region the PET is stretched, blown and biaxially oriented. Both of the above mentioned regions are resistant to the unrelaxed retractive stresses. The center amorphous region is resistant due to its increased thickness and the uniformly oriented region because of its uniform biaxial orientation. However, between the amorphous and uniform regions there exists an inevitable artifact of the blow molding process, a transition region. The transition is neither significantly thick nor uniformly oriented, and therefore, not resistant to the unrelaxed retractive stresses.
U.S. Pat. No. 4,598,831, issued to Nakamura, discloses a method of reshaping and orienting a portion of the transition region in the base so as to form a radial array of outwardly directed triangular pyramid sections, with the bottom surface of the pyramid sections being sufficiently stretched and oriented. However, between the pyramid sections, and along the sidewalls of the pyramids themselves, there exists a significant amount of transition region which remains unoriented, and therefore, non-resistant to the retractive stresses.
The U.S. patent application entitled "SPIRAL CONTAINER BASE STRUCTURE FOR HOT FILL PET CONTAINER", Ser. No. 452,638, filed Dec. 19, 1989 and assigned to the assignee of the present invention, discloses a PET container having a spiral base structure and is herein incorporated by reference.
In accordance with the present invention, a PET container is provided having a base structure of a configuration which maintains both structural rigidity and resistance to random deformation and shrinkage in response to the previously mentioned mechanical and thermal stresses. This is accomplished by providing selective sectional reinforcement where the greatest concentration of unrelaxed retractive stresses exist, namely, in the transition region. More particularly, a number of specific sized involute embossments are positioned in that limited transition region of the base structure's dome. The embossments are both effective and efficient, in that, they are provided only where required and their specific geometry is tuned to the stresses particularly encountered in the thermal stabilization process.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.